Basically it is pretty common practice and I cant judge a casino ask for KYC after big wins as scam, the main point should be after the KYC is verified, what the casino do?
If they process withdrawal after verified KYC without too many drama, I will not call the casino as scam but if the casino do something like too many excuses to delay or even to make players unable to withdraw the big win with unclear reason, I will call it scam.
So the point is not about when the casino ask KYC because most casino has written terms, usually it is written as "we reserve the right to ask KYC at anytime..." or something similar.